The present invention relates to bearing arrangements, particularly for apparatus utilized in machine tool and metal forming environments, such as machine tool slides, tables and carriages.
In machine tools, it is often necessary to traverse either a tool or a workpiece over fairly long linear distances during machining of the workpiece or between machining steps. For example, a workpiece may be rigidly secured to a machine tool table, and then the table traversed along a linear distance by means of a ball screw or the like to move the workpiece relative to a tool, such as a rotating milling cutter, which may cut a groove or chamfer in the workpiece. Conversely, the workpiece would be clamped to a stationary support and the tool moved relative to it during machining, as by a carriage or slide. In other machining operations, such as sequential hole boring, both the workpiece and tool carriage may remain stationary during machining, with a rotating bit moved into the workpiece by means of an axially movable spindle. In this case, the bit is withdrawn and then either the workpiece or tool moved to the next location for boring of the subsequent hole.
In each of the examples outlined above, it is important that the workpiece or tool be moved with extreme accuracy so that there is virtually no component of movement in directions perpendicular to the primary axis of movement. Obviously, a lack of stiffness in directions perpendicular to the primary direction of movement would result in non-linear machining in the case of the first two embodiments, and would result in improper relative placement of the bored holes in the case of the latter embodiment.
In the past, the table or carriage supporting the workpiece or tool was held in alignment by means of a V guide groove within which a correspondingly shaped ridge would be received, and the table prevented from skewing by virtue of the weight of the table forcing the complementary surfaces into mating engagement. In other cases, the table would be supported for movement by means of hydrostatic bearings mounted in either the table or support base, and which developed high pressure oil films to reduce the frictional drag between the table and support base.
Although such a table may have acceptable accuracy under static conditions, once a tilting force is applied to the table, as by high cutter force, the static weight of the table can be overcome by the moment arm of such force, thereby disrupting the accurate mating of the guiding surfaces.
Although hydrostatic bearings are very effective for reducing frictional drag between surfaces moving relative to each other, the effectiveness of the bearings are very sensitive to the clearance between the surfaces. When the clearance increases, the pressure of the hydraulic fluid necessarily decreases unless the overall hydraulic pressure and flow of the system is correspondingly increased. Furthermore, each time a different workpiece or tool weight is placed on the table or the force from a tool acting against the workpiece increases, the thickness of the oil film will change. If the oil film does change, then the tolerance is degraded by that amount. In other words, the oil film is dependent on a known preload under static conditions, and each time the preload changes, the thickness of the oil film will also change thereby increasing the tolerance of the apparatus.
Although very accurately machined guiding surfaces can be obtained for short traverse distances, the machining tolerance becomes much more difficult to obtain for very long traverse distances, such as distances of ten feet, for example. In the case where the movable element is sandwiched between two guiding surfaces, it is not only necessary for the surfaces themselves to be extremely flat, but they must be perfectly parallel to each other so that the clearance for the hydrostatic or antifriction bearings will be maintained constant. For long traverse distances, such parallelism is virtually impossible to obtain. The problem is further complicated by the necessity for constraining the movable element, such as a workpiece table, in two orthogonal directions. Here, two true, flat and parallel pairs of surfaces must be machined along the entire length of traverse of the table or carriage.
It is also known to support a movable element, such as press slide, between opposing pairs of hydrostatic bearings. Although this bearing arrangement resists, to some degree, lateral deflection of the movable element as it moves or reciprocates along its axis of movement, the hydraulic bearing clearance is disrupted by thermal expansion. Moreover, the proper clearance for the hydraulic film must be maintained on both sides of the movable element, and this requires extremely flat, true, and parallel opposing surfaces in the cases where high accuracy is required. As indicated earlier, maintaining such machining tolerances over long traverse distances is very difficult to achieve.
Preloaded hydrostatic bearings are also known, but the oil film left on the exposed ways in long traverse environments, attracts dirt and metal chips, thereby interferring with the accuracy of the guide surface.
Antifriction bearings, such as roller and ball bearings, can be utilized in bearing arrangements where very high accuracy is necessary, because they will deflect to a known degree with a certain known preload. The difficulty, however, is applying a preload to the bearings which is generally constant over the entire length of traverse of the table, carriage or other movable element. One prior art technique for preloading such bearings is to utilize a mechanical spring device, which applies a known amount of pressure to the bearing. The problem with such a device is that the preload applied by it is only correct for a given clearance, which may occur in only one position of the table or carriage. For example, if the opposing surfaces between which the table or carriage moves are non-parallel so that their separation differs from one position of the table or carriage to another, the spring device will be applying a different preload to the bearing. This is because of Hook's Law whereby the force exerted by a spring is proportional to its deflection.
One of the problems with hydrostatic bearings and other opposed bearing arrangements is the necessity for maintaining the parallelism of the surfaces against which the respective bearings bear within a relatively narrow range. If the surfaces are not parallel, the clearance for the hydrostatic bearing will increase or decrease as the movable element traverses thereby requiring a higher or lower hydraulic flow, respectively, to maintain the same preload on the other bearing. When the preload changes, the deflection of the antifriction bearing changes or the oil film of the hydrostatic bearing changes thereby changing the position of the movable element relative to the true surface, which defines the reference plane for that degree of freedom. The problem of maintaining parallelism between the two surfaces increases as the length of traverse of a movable element increases. For very long traverse distances, such as those of ten feet, for example, it is almost impossible to maintain surfaces which are always parallel.